There is much interest in developing a microwave microscope that can measure one or more electrical characteristics of a sample in the gigahertz range and, by scanning the probe over the sample surface, to image the spatial variation of such characteristics. Such a microwave microscope would be very useful in the semiconductor industry for mapping resistivity and dielectric constant over the wafer, particularly during its fabrication since a microwave measurement can be non-destructive. In some instances, the thickness of a layer may be related to such electrical characteristics. The gigahertz measurement frequency corresponds to the important frequencies utilized in semiconductor devices. The probe of such a microwave microscope can also be used as a read head for nano-scale information storage on ferroelectric recording medium.
For integrated circuits, the imaging resolution must be on the order of less than a few microns since feature sizes are being pushed to much less. However, microwave wavelengths and waveguide dimensions are in the range of centimeters to millimeters, far greater than the desired resolution.
Several proposals have been made for microwave probes that have a spatial resolution much less than the wavelength of the radiation being used, using a technique called near-field. This technique allows spatial resolution less than the wavelength being used by scanning a probe very close to a sample. For example, Xiang et al. in U.S. Pat. No. 5,821,410 describe a sharpened probe tip extending through an aperture in a resonant quarter-wavelength cavity and projecting toward the sample under test. Anlage et al. in U.S. Pat. No. 5,900,618 disclose a somewhat similar microwave microscope.
Somewhat similar measurements can be made using a scanning capacitor measurement apparatus with a small tip electrode and the sample acting as the other electrode, such as disclosed by Williams et al. in U.S. Pat. No. 5,523,700, by Slinkman et al. in U.S. Pat. No. 5,065,103, and by Matey in U.S. Pat. No. 5,581,616 and reissued U.S. Pat. Re. 32,457. Calculations relate the measured capacitance some measurement parameters such as DC voltage with electrical characteristics of the material. This design is a non-resonant structure, thus can have a broad bandwidth of operation. The sense area of these designs however extends far from the probe electrode, and it is difficult to relate the measured impedance to the dielectric constant and resistivity of the material.
Kelly et al. in U.S. Pat. No. 6,825,645, incorporated herein by reference, discloses a microwave imager, which utilizes a non-resonant structure to gain a broad bandwidth of operation and further puts a grounded electrode next to the sensing electrode, which avoids the problem of a large sense area.
These proposals, whether using a resonant structure or a non-resonant structure, all depend upon a single electrode to stimulate the sample and to sense the electrical potential change on the sample surface. Thus, there is often a large reflected excitation signal on the electrode which has not interacted with the sample and which is larger than the sensed signal which has interacted. The reflected signal may exist exists even when no sample is present. This reflected signal is referred to as the common mode signal. In an attempt to detect a small signal emitted from a sample by amplifying the signal from the probe, the common mode signal can easily saturate a detector. A common mode cancellation circuitry can be used to cancel the common mode signal. However, such a circuitry is not always stable, and it adds another source of shot noise to the original shot noise in the common mode signal.